There are two possible encodings for the message content: one using MIME, the other using uuencode. MIME is the preferred encoding. In addition, MAPI defines a per-recipient property, PR_SEND_RICH_INFO, that governs whether or not TNEF information should be included in an outgoing message. So there are a total of four ways of encoding message content:
How to choose MIME or uuencode for outbound messages is not specified.
The following properties are excluded from TNEF: PR_SENDER_*, PR_ATTACH_DATA_*, PR_BODY. All other transmittable message properties are included in the TNEF stream.